Unit 13 Processing by Heat Application

Processing and Preservation UNIT 13 PROCESSING BY HEAT APPLICATION

Structure

13.0 Objectives 13.1 Introduction 13.2 Effect of Heat on Texture and Composition 13.3 Effect of Heat on Microorganisms and Enzymes 13.4 Role of Heat Application – Peeling, Juice Processing, Syrup / Brine Preparation & Filling Use of Heat in Peeling of Fruits and Vegetables Use of Heat in Juice and Pulp Processing Syrup / Brine Preparation and Filling 13.5 Blanching and Exhausting Blanching Exhausting 13.6 Pasteurization and Sterilization Pasteurization Sterilization 13.7 Combination of Time, Temperature, pH/Acidity Time and Temperature pH or Acidity 13.8 Role of Heat Application during Product Preparation Use of Heat in Jam and Jelly Production Use of Heat in Ketchup and Sauce Production 13.9 Let Us Sum Up 13.10 Key Words 13.11 Answers to Check Your Progress Exercises 13.12 Some Useful Books

13.0 OBJECTIVES

After reading this unit, you should be able to:

• define types of heat applications; • describe the role of heat application during processing and product preparation; • state the effect of heat application on quality of food, microorganisms and enzymes; and • discuss the effect of heat in combination with time, temperature, pH or acidity.

13.1 INTRODUCTION

Fruits and vegetables are living entities and highly perishable horticulture produce. Enzymes and microorganisms can easily spoil these commodities. In order to keep fruits and vegetables for a longer period, they are protected by several means. Heat application is one of the most important methods of preservation of fruits and vegetables. The main purpose of processing of fruits and vegetables by application of heat is to inactivate enzymes and killing of microorganisms. Heat can be applied in different forms such as blanching, pasteurization and sterilization. Heat application influences the texture and composition of fruits and vegetables, and microorganisms and

Processing by Heat Application enzymes. Heat application plays a great role during various unit operations of processing of fruits and vegetables viz., peeling, preparation of juice and pulp; syrup and brine, during blanching, exhausting, pasteurization or sterilization. There are certain factors like time, temperature; pH and acidity in combination with heat application will affect the quality of the processed products. There are several types of processed products, generally preserved by heat application such as jam, jelly, juices, sauces, candies, canned and bottled products etc. Heat processing has advantages such as to improve eating quality and availability of some nutrients and a simple control of processing conditions.

13.2 EFFECT OF HEAT ON TEXTURE AND COMPOSITION

Carbohydrates, proteins, fats, vitamins, minerals and fibre are the major dietary constituents of fresh vegetables in addition to a large quantity of water. Fruits are held in high public regard as sources of wholesome food and are valued for their taste, aroma and texture. Fresh fruits appeal virtually to all the senses, smell, taste, touch, sight and even sound as when one bites into crunchy apple. Vitamins and minerals are the major contributions of fruits to the human diet. Although some fruits are also considered good energy sources and some may contribute notable amount of fat (e.g. Avocados and nuts), sugar (e.g. Dates and figs) and proteins (e.g. tucuma). Fruits may play an important role in the diet by supplying fibre. Heat processing is one of the most important methods for extending the storage life of fruit and vegetable products. Because of this extended life, the processed products are made available throughout the year. This has increased the availability of nutrients to the consumer. However, heat processing also has a detrimental effect on nutrients since there is thermal degradation of nutrients. Therefore, heat processing makes it possible to extend and increase availability of a food product to the consumer, but the food product may have a lower nutrient content compared to the fresh food product. This is a great challenge to the food processing industry is to minimize the loss of nutrients during thermal processing. Heat processes are used in terms of blanching, pasteurization and sterilization. The primary objective of heat processing is to increase the palatability of the food and to increase the storage life of food product besides minimizing food- borne diseases. For example, the cooking, which includes roasting, boiling, frying etc. The heat processes viz. blanching, pasteurization and sterilization has already dealt in Para 2.5 and 2.6 in a greater details, here we are discussing their effect on texture and composition of fruits and vegetables. Heat processing generally includes the terms blanching, pasteurization and sterilization and their principal purpose is given here. Blanching is a heat process frequently applied to fruit and vegetables prior to freezing, drying or canning. The objective of the blanching process depends on the subsequent treatment of the foodstuffs. The adequacy of the blanching operation is enzyme inactivation. Generally, if enzymes are inactivated, the heat treatment was sufficient to accomplish the objectives of blanching prior to canning. Pasteurization is a heat process meant to inactivate part, but not all, of the vegetative microorganisms present in the food product. Since the food is not sterile, the pasteurization must also be used in conjunction with other

Processing and Preservation preservation techniques like fermentation, refrigeration, and maintenance of anaerobic conditions. Sterile is a term, which used to a condition in which no viable microorganisms are present. A viable organism being one that is able to reproduce under conditions optimum for its growth. Sterilization is a term used to apply to a heat process that produces a sterile condition in food product. Some microorganisms and their spores are extremely heat resistant and cannot be destroyed completely. Severe heat treatment can only make the food sterile, but it will influence the organoleptic and nutritive value of food. Therefore, the sterilization process is also used in conjunction with other preservation techniques, such as packaging and control of storage temperature. Thus, the remaining dormant microorganisms or their spores will not grow in the environment of the food under the storage. Foods that have been thermally processed and meet this requirement are said to be “commercially sterile”. Heat processing may change not only the chemical composition of the food but also its structure by softening tissues, releasing or tying up moisture, destroying or forming colloidal suspensions, gels or emulsions. Protein may become denatured and therefore more available to some organisms than it was in the native state. Starch or protein may become gelated, releasing moisture and becoming more easily decomposed. For the same reasons, cooked food usually is more easily decomposed than the original fresh food. Preservation is a convenient method of storing fruit for use in periods when the fresh products are not available. The characteristics of fruit are usually altered to such an extent during processing that the processed products do not necessarily resemble the fresh products. If processed and stored properly, the nutritive value of these processed fruit is comparable to that of the fresh products. The thermal processes viz. blanching, pasteurization and commercial sterilization have to be optimized for nutrient retention, given in the box below:

Process Method of optimization

Blanching Thermal losses, leaching losses, oxidative degradation, damage to product Pasteurization HTST if heat resistant enzymes are not present Commercial Convection – heating foods and aseptic processing. Sterilization HTST until heat-resistant enzymes become important Conduction heating foods; not necessarily HTST; difficult but not impossible calculation.

Effect of blanching on nutrients The effect of blanching on food nutrients is generally considered based on thermal, leaching and oxidative losses. Blanching is done either by dipping in hot water or exposing to steam. In water blanching the loss of water –soluble vitamins increases with contact time, and fat – soluble vitamins are relatively unaffected. Steam Blanching results in greater retention of water – soluble nutrients than water blanching. There is a slight improvement in ascorbic acid retention with IQB (individual quick blanch) as compared to conventional steam blanching. In conclusion, blanching can significantly reduce the nutrient

Processing by Heat Application content of foods. The extent of losses of nutrient is dependent on the blanching method and the product. Effect of pasteurization on nutrients The foods having a low pH (mostly below 4.5) are generally pasteurized. Some food products, which receive pasteurization, are summarized in the box given below: Food Products preserved by Pasteurization treatments

Sl. Product Pasteurized Temperature (oC) No.

1. Carbonated Beverages 60-65 2. Non Alcoholic fruit drinks 65-70 3. Dill Pickles, Carbonated fruit juices 70-75 4. Apple juice (Holding process) grape juice, 75-80 raspberries, strawberries in syrups in cans or jars 5. Processed and pickled vegetables, wine (U.S.), 80-85 desiccated coconut 6. Apple juice (flash process), canned olives, 85-90 citrus juices, peeled tomatoes (pH 4.1) 7. Tomato puree, citrus juices (flash process) 90-95 tomato juice, peeled tomatoes (pH 4.5), Jam 8. Wine (flash process), fruit puree, fruit juices, 95-100 canned fruits (internal can temp.) Source: Shapton et al. (1971) Since most of the heat – labile nutrients are relatively stable in acid conditions; nutrients losses in those products are relatively minor. Although thermal losses during pasteurization may be small, oxidative losses can be high. We have already discussed that HT ST (High Temperature Short Time) process results in greater nutrients retention for those nutrients affected by the pasteurization process. They are primarily thiamin, vitamin C and vitamin B 12. The nutrients that are more sensitive to high temperature are generally the same ones that are of concern during storage. It would be reasonable that the lower the storage temperature, the slower the rate of nutrient degradation. Effect of commercial sterilization The destruction of nutrients the thermal process is dependent on i) time- temperature treatment used as the basis of the process and ii) rate of heat transfer into the product. The commercial developments have focused primarily on increasing the rate of heat transfer into the product. Hence, agitated retorts such as the orbitort, steritort, flame sterilizer and hydrostatic cooker have been developed. In addition to increasing the rate of heat transfer, however, there also has been a gradual shift to higher processing temperature. This results in adaptation of HTST process for greater nutrient retention in those products heating

Processing and Preservation primarily by convection. It has been observed (Ammerman 1957) that retention of vitamin C in tomato juice is improved when processing is conducted at a HTST condition. The HTST process is particularly adaptable to aseptic processing. In this system, processing temperatures in excess of 149oC are used for very short periods i.e. order of seconds. Under these conditions nutrient retention may be greatly enhanced. HTST aseptic canning also results in a significant improvement in organoleptic qualities i.e. colour, taste and aroma. In an evaluation of HTST aseptic processing, it was found that thiamin retention was significantly greater in HTST products than in conventionally canned and retorted products. Based on existing literature available, nutrient losses range from 0 to 91 percent, depending on the nutrient and product. It is a misconception to think that commercially sterile products remain unchanged during storage. This is not the truth. Organoleptic and nutrient changes do occur during storage, the extent of the changes being dependent on the time and temperature of storage. It has been observed that low temperature storage results in an improvement in nutrient retention. In conclusion, it is evident that there is a significant loss of nutrients during canning and that these losses increase during storage. Altering heat processing and storage conditions to maximize nutrient retention is an important and necessary consideration for the fruit and vegetable processing industry.

13.3 EFFECT OF HEAT ON MICROORGANISMS AND ENZYMES

Generally the food spoilage takes place at any temperature between –5o and 70oC. Since microorganisms differ so widely in their optimal, minimal and maximal temperature for growth, it is obvious that the temperature at which a food is held will have a great influence on the kind, rate and amount of microbial induced change that will take place. Even a small change in temperature may favour an entirely different kind of organism and result in different type of spoilage. For example, moulds and yeasts generally do not grow well above 35 to 37°C, hence not important in foods held at high temperatures. But moulds and yeasts grow well at ordinary room temperatures and many of them grow fairly well at low temperatures. Most bacteria grow best at ordinary temperatures. Some (thermopiles) grow well at high temperatures and others (psychrotopes) at chilling temperatures. The heat, which kills microorganisms and inactivate enzymes is supposed to be denaturation of the proteins. The heat treatment varies depending upon the different factors such as kind of organism, its state and the environment during heating. Depending upon the heat treatment employed, only part of the vegetative cells, most or all of them may be killed. There are known certain terms of heat treatment given below: Heat Processing – Use of high temperature to destroy enzymes and microorganisms that could reduce quality and /or safety of food. UHT – A very severe heat treatment, very short in time. Pasteurization – A mild heat treatment used primarily to destroy pathogenic organisms but it also destroys enzymes and reduces microbial load. Requires an addition preservation method to extend shelf life (for example – refrigeration, drying).

Processing by Heat Application Commercial sterilization – A severe heat treatment that destroys pathogenic and many microorganisms that could spoil food. Extends shelf life, room temperature stable. Sterilization – A very severe heat treatment that destroys all microorganisms. Details of these heat treatments can be seen in other Para of this unit.

Factors affecting heat resistance There are certain factors, which are known to affect the heat resistance of cells or spores of microorganisms. This point must be kept in mind when microorganisms are compared and when heat treatments for destruction of an organism are considered. The main known factors are listed below: 1. The temperature – time relationship – The time for killing cells or spores under a given environment decreases as the temperature is increased. 2. Initial Concentration of Spores – the more the spores present, the greater is the heat treatment required to kill all of them. 3. Environment of Vegetative spores – i) Medium of growth: Nutrients in the growth medium influences the heat resistance. The better the medium for growth, the more resistant the spores cell. ii) Temperature of incubation: In general, resistance increases as the incubation temperature is raised toward the optimum for the organisms. iii) State of growth and age: Very young immature spores are less resistant than mature ones. iv) Desiccation: Certain spores are harder to kill by heat than those kept moist. 4. Composition of substrate also influences the thermal death time of the microorganisms. i) Moist heat is a much more effective killing agent than dry heat. ii) pH: In general microbial cells or spores are most heat – resistant in a substrate near neutrality. (See more details in 2.7 Para) iii) Other constituents: Common salt and sugar also protect some organisms or spores. The colloidal materials, especially proteins and fats, are protective against heat. Heat resistance of microorganisms The heat resistance of microorganisms usually is expresses in terms of their thermal death time. The thermal death time (TDT), which is defined as the time it takes at a certain temperature to kill a stated number of organisms (or spores) under specified conditions. Most of the yeasts, moulds and their spores are killed by moist heat at 60°C, but some species are considerably more heat resistance, but may not survive 100°C. The bacterial vegetative cells are easily killed at 80-90°C. But resistance of bacterial spores at 100°C may vary from less than 1 minute to over 20 Hours.

Processing and Preservation Heat resistance of enzymes The most food enzymes are destroyed at 80°C, some may with stand higher temperatures, especially if high temperature – short –time heating is employed. Generally the effect of heat is intended for preservations of food i.e. to kill bacteria and inactivate enzymes. For example, the pasteurization of milk, fruit juice and beverages and the sterilization of canned foods. The main aim is to deliver the required microbial kill with as little damage to the structure and consumption of the food products as possible. # Check Your Progress Exercise 1 Note: a) Use the space below for your answer. b) Compare your answers with those given at the end of the unit. 1. Named constituents of fruits and vegetables. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 2. What is the primary objective of heat processing? ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 3. What are the methods of optimization of heat process? ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 4. Define different terms of heat treatments. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………

Processing by Heat Application ……………………………………………………………………………… ……………………………………………………………………………… 5. List the factors affecting heat resistance. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………

13.4 ROLE OF HEAT APPLICATION − PEELING, JUICE PROCESSING, SYRUP/BRINE PREPARATION & FILLING

13.4.1 Use of Heat in Peeling of Fruits and Vegetables Generally fruits and vegetables are peeled by hand, mechanical means, use of heat and lye solution. Heat in various forms is applied in last two methods of peeling. Heat is generally used in the form of boiling water, steam, roasting by a direct gas flame and heating in steam under pressure. Peaches, tomatoes, beetroots are either immersed in boiling water or exposed to live steam until it loosen skin or cracked or peel is separated from the flesh easily, and then cool by spraying or immersion in cold water. The loose/ cracked or separated skin is easily slipped from the fruit by hand or by spraying water. Sweet potatoes are steamed under pressure to soften the skin and are peeled by hand. Sometimes peeling of carrots, beets, potatoes and apples and some other vegetables can be done by heating under pressure to a very high temperature for a few seconds then releasing the pressure. This method of peeling can be a continuous or batch type process. Pimientos and onions are flame-peeled in a continuous natural gas-heated chamber at very high temperature (1568°C). Then they are washed in sprays of water. Uniform size of the produce gives better results. Heat is also used during lye peeling. It makes the lye solution more effective than cold lye. Lye is a solution of alkali such as sodium hydroxide. Its strength and time of immersion of the commodities depends upon type and nature of fruits and vegetables and thickness of their skin. Lye peeling is used on peaches, sweet potatoes, apricots and carrots. Sometimes the sodium carbonate-sodium hydroxide mixture is also used, is called canners alkali. The lye solution having 1.5 to 2 percent sodium hydroxide is used for peeling of fruits. For vegetables 10 to 15 percent sodium hydroxide solution is used. The time of immersion in lye solution for fruits varies from ½ to 2 minutes and 6-8 minutes for vegetables. The boiling dilute lye solution causes the separation of the outer skin of the fruits from the flesh. In case of sweet potatoes the action of lye solution is upon the cutin. 13.4.2 Use of Heat in Juice and Pulp Processing

Processing and Preservation Use of heat application helps in clarification and preservation of fruit juices and beverages. Fruit juices after pasteurization will become clear during storage. The length of storage required depends upon the type of juice and other conditions. For example the pomegranate juice will become clear within twenty four hours, after pasteurization where as grape juice requires several months for setting solids and it can be enhanced by refrigerated low temperatures. The heat treatment is used to pasteurize the fruit juices and it means the destruction of all microorganisms capable of increasing in the juice of causing spoilage. It doesn’t kill the spore forming bacteria such as Bacillus subtilis and Bacillus mesentericus but these spore bearing bacteria cannot grow in acid fruit juices. However, pasteurization should be done at such a temperature and for such a time that yeast and moulds are destroyed. For example yeast is killed in a few minutes by heating juice at 60-66°C. But resistant mould spores require a temperature of 800C for 20 minutes; however, carbonated juices can be pasteurized safely at 66°C. Thus, we can conclude that the fruit juices must be pasteurized at lower temperature (71-74°C). The presence of carbon dioxide generally does not reduce the death temperature of fruit juice microorganisms such as yeast, mould spores, and bacteria. But it prevents the growth of surviving mould spores. However, it has been observed that carbonated and heavily inoculated fruit juice heated for 30 minutes at 65°C, which prevents subsequent development of mould spores. The bulk juices are pasteurized either as continuous or batch type process. The continuous pasteurization is done in a tubular or plate type heat exchangers. Hot water or steam is used as heating medium. The batch type pasteurization is done in steam-jacketed kettle or in a tank equipped with steam coils. In these vessels, the juice is placed and heated to the desired temperature. This system has some disadvantages such as the small portion of juice may be over heated or exposed to air and cause oxidation during pasteurization and injury to colour and flavour on prolong heating. Heating can also be done by electricity. The fruit juices are passed between carbon electrodes. The juice is heated instantaneously to the desired temperature when ordinary alternating current (110 Volts 60cycle) is passed in the electrodes. Heat is generated by passage of current against the resistance of juice, without heating the electrodes. The fruit juices are also preserved by flash pasteurization and cooling. The prolong heating results in considerable injury to the flavour and colour of the products. Hence, the fruit juices are heated at higher temperatures and rapidly cooled and then under aseptic conditions filled into sterile containers such as bottles and sealing by sterile cork or caps. Great care must be taken to avoid the infecting juice with mould or yeast. For example the juice is heated to about 82 to 90°C for a few seconds only, the juice suffers very less injury to colour or flavour. In some instances the citrus, apple or tomato juice is flash pasteurized momentarily at 116°C or higher, but cooled to 98°C (tomato juice) and 88°C to other juices before filling into the container. This treatment will inactivate enzymes also. Now a day’s pasteurization is generally done by “hot- fill-hold-cool” method. In this method the hot sterile bottles or cans usually filled hot directly from bulk or continuous pasteurizer, sealed and given no further heating but cooled in the air and water, respectively. Heat should also be used to sterilize bottles, cans, and bottle caps otherwise they may cause infection in bottled beverages.

Processing by Heat Application 13.4.3 Syrup/Brine Preparation and Filling The syrups are added to fruits and brines are added to vegetables during canning. These additives improve the flavour; fill the spaces between the pieces of canned fruits and vegetables and aid in the transfer of heat during the processing. For making syrup, cane sugar, liquid glucose or invert sugar is used but usually cane sugar is employed. The sugar is dissolved in small quantity of water to yield heavy syrup (60-650Brix). The cane sugar and water are heated together until a clear syrup is obtained. Heat treatment is provided through steam. Tanks with steam heated coils or steam jacketed kettle is used for the preparation of the syrup. Sugar syrup is clarified by passing through cloth. Then it is diluted to the desired degree Brix depending on the nature and grades of the fruit. Here the heating helps in dissolving sugar in the water and quick filtration before it is cooled. Hot syrup is filled into cans containing fruit pieces, which helps in reducing time of exhausting. Dilute brines of 1 to 2 percent common salt are used during canning of vegetables. Salt used for canning should be at least 99 % sodium chloride (NaCl) and lower purity less than 98 % should not be used. Salt is dissolved in water by heating, after proper filtration is filled into the cans containing prepared vegetables before exhausting. Hot brine is used for filling into the cans.

13.5 BLANCHING AND EXHAUSTING

13.5.1 Blanching Blanching is a unit operation applied to fruits and vegetables prior to canning, drying or freezing. Blanching is a mild type of heat processing. It involves heating food to preset temperature for preset time. It is usually done in water or steam at a temperature less than 100°C. Prepared fruits and vegetables is kept in hot water or exposed to steam and then cool rapidly to ambient temperature. Blanching is used to destroy microorganism and enzymatic activity in fruits and vegetables. Blanching caused inactivation of enzymes in canning, freezing and dehydration, because freezing and dehydration are insufficient to inactivate enzymes. Sometimes canning process may allow sufficient time for enzymatic activity. And under blanching may increase the enzymatic activity. There are four types of enzymes such as lipoxygenase, polyphenololase, polygalacturonase and chlorophyllase, which causes loss of quality in fruits and vegetables, must be inactivated. The enzymes such as catalase and peroxidase are heat resistant, which need appropriate time and temperature to inactivate them. Blanching also reduces the number of microorganisms. Blanching of fruits and vegetables in steam has the advantages such as less loss of water-soluble constituents, less volume of waste, easy to clean and sterilize. But it has some disadvantages i.e. higher capital costs, uneven blanching, and low efficiency. Generally hot water blanchers are used because of lower capital costs and better energy efficiency. But it has some disadvantages like loss of water- soluble constituents, risk of contamination and higher cost of water and disposal of effluent than steam blanchers.

Processing and Preservation Blanching of green leafy vegetables especially spinach at boiling point causes loss of green colour but lower temperature (77°C), it retains the natural green colour, even when heated at higher temperature (121°C) during sterilization. At lower temperature time the enzyme chlorophyllage remains active for little time and convert chlorophyll to a phyllin, which retain green colour.

13.5.2 Exhausting Exhausting is a mild type of heat processing. It is one of the unit operations involved during canning of foods. Exhausting, generally remove air and gases from cans. It can be done either by application of heat. In this method, the cans are passed through a tank of hot water or exhaust box under steam. The temperature of water is generally ranged between 820C to 1000C and the center of the can should reach a temperature of about 790C. The time of exhausting varies from 6 to 10 minutes, depending upon the nature of the commodities. Generally, exhausting of the cans is preferred at lower temperature for a longer time to ensure to uniform heating of the contents, without softening of the produce. Exhausting at high temperature should be avoided, otherwise it will be formed more volume of water vapour, which may produce greater vacuum in the can. Exhausting help in avoiding the corrosion of the tin plate and pin holing during storage, minimizing discolouration by preventing oxidation, better retention of vitamin C, prevents bulging of cans when stored at high altitude. It also helps in reducing chemical reaction between containers and food and prevents development of expressive pressure and strain during sterilization of cans.

13.6 PASTEURISATION AND STERILISATION

13.6.1 Pasteurization Pasteurization is a mild type of heat processing. In this method, the heat treatment is performed below 100°C, which kills some selective but not all of the microorganisms present. Heating is done in form of hot water, steam, dry heat or electric currents. The food products are cooled rapidly to ambient temperature after heat treatment. The main function of pasteurization is to inactivate enzymes, kill non-spore forming bacteria, yeasts and moulds. Pasteurisation is used in combination of other preservation methods such as fermentation (pickles), refrigeration and anaerobic conditions as the food products are not sterile. Pasteurization is generally used in fruit juices, where the environment is not suited for the growth of spoilage causing microorganisms. The severity of pasteurisation depends on pH of the food products. In low acid (pH>4.5) foods, the destruction of pathogenic bacteria, whereas in high acid foods (pH<4.5), the destruction of spoilage microorganisms and inactivation of enzymes are essentially the requirement. Heating to 100oC mostly preserves the fruit juices or slightly below for a sufficient time to kill spoilage microorganisms are called pasteurization. Generally, the fruit juice is hermetically sealed in cans or glass bottles before being pasteurized. The juice would not spoil as long as the cans or bottles remain sealed against the entry of spoilage microorganisms from outside. Some spores and spore forming bacteria like Bacillus subtilis and Bacillus mesentericus can survive the process. But these organisms are highly

Processing by Heat Application sensitive to acid products and cannot grow in acid fruit juices and beverages. Generally, the fruit juices are pasteurized at about 850C for 25 to 30 minutes depending upon the nature of the juice and the size of the containers. In this process most of the bacteria, mould, spores and yeasts are readily killed. Fruit juices are pasteurized by two methods. First by heating the juice at a low temperature for a long period, and secondly by heating the juice at high temperature for a short time (HTST). Pasteurisation is governed by the temperature and its types & methodology. For example under ‘Holding’ pasteurisation also called Bottle method, the prepared and finished juice is filled into glass bottles leaving proper head space. The bottles are then sealed airtight and pasteurized. The headspace is left in the bottle for the expansion of the juice during heating. In over-flow method of pasteurization, the fruit juice is heated to a temperature about 2.50C higher than the pasteurisation temperature. The hot juice is filled into hot sterilized bottles upto brim and sealed by crown corking. A care is taken that the temperature of the juice does not fall below pasteurisation temperature during filling and sealing. In flash pasteurization the juice is heated rapidly to a temperature of 5.50C higher than pasteurisation temperature for one minute and filled into bottles and canes, sealed air-tight under cover of steam and then cooled. The steam will sterilize the seal. The flash pasteurisation has some advantages, that it helps in minimizing loss in flavour, retention of vitamins, it effects economy in time and space, keep the juice uniformly cloudy, heats the juice uniformly, which reduces to a minimum any cooked taste of the juice. Flash pasteurisation is a type of higher temperature and short time (HTST) heating system. Some liquid foods are pasteurized after packaging. Food packaged in glass containers is generally pasteurized with hot water. The unpackaged foods such as fruit juices and beverages of low viscosity are pasteurized in continuous operation using plate heat exchangers.

13.6.2 Sterilization Sterilization is a process of heat application above 100°C, which is employed to deprive microorganisms of their ability to multiply. We can obtain this sterilization temperature by using steam under pressure. The boiling point of water can be raised if the water and steam are enclosed in a strong retort (autoclave). In this retort, temperature increases with increase in steam pressure. The temperature at sea level is 100°C without pressure but at 0.7kg/cm2 steam pressure the temperature of 121°C or above may be easily obtained. At these high temperatures the spores of the heat resistant bacteria are quickly killed. The longer is the heat treatment time at lethal temperatures, the larger is the number of killed microorganisms. At higher temperature, the shorter is the time required to kill microorganisms and lower is heat-included damage to food products. Theoretically, absolute sterility does not exist. In commercial practice not all cans of food are sterile. However, they usually do not spoil because conditions in the container are not favourable for the growth of concerned microorganisms. The pH may be too low or absence of oxygen. Therefore, the term processing is highly suitable than the term ‘sterilization’ applied to canned foods. The foods products low in acid and often high in protein and contain spore- forming bacteria are difficult to sterilize. The acidity of fruits and tomatoes

Processing and Preservation greatly lower the death or sterilizing temperature, which usually explains why acid fruits are easily sterilized. The purpose of sterilization is the destruction of all pathogenic, spoilage causing organisms and non-pathogenic microorganisms. This process will make the treated product safely preserved at room temperature. Thus, the food products with safe preservation at room temperature are generally defined as commercially sterile. # Check Your Progress Exercise 2 Note: a) Use the space below for your answer. b) Compare your answers with those given at the end of the unit. 1. Describe types of heat treatments. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 2. What is the difference between pasteurization and flash pasteurization? ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 3. Explain sterility and commercial sterilization. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………

13.7 COMBINATION OF TIME, TEMPERATURE, pH/ ACIDITY

13.7.1 Time and Temperature

Processing by Heat Application The food is generally decomposed by the presence of microorganisms and can be prevented by killing them. The heat preservation is one of the most important methods of killing / inactivation of microorganism as well as enzymes which are already present in the food. Heat application kills the microorganisms because of denaturation of proteins. Different microorganisms have different heat resistance. This can be shown by their thermal death time (TDT) at a particular temperature. For example the TDT of Lactobacillus bulgaricus is 30 min. at 71°C; Escherichia coli is 20-30 min. at 57.3°C and Streptococcus thermophylus is 15 min. at 70-75°C. Thus it is the combination of time and temperature required to kill the specified microorganisms. In a set of given conditions, with increase of temperature the time required to kill cells or spores decreases. The canning is one of the major methods of food preservation based on destruction of microorganisms by heat treatment and prevention of recontamination. The heat treatment in respect of time and temperature is governed by the nature of food product and size of the container. Thus we can enlist below certain factors, which affect the heat resistance of cells or spores. i) Initial concentration of spores ii) The time-temperature relationship iii) The type and nature of microorganisms iv) Composition of substrates such as moisture content, pH or acidity, fat, protein, pectin, etc. The heat treatment based on temperature can be grouped as i) Pasteurization (heating below 100°C) ii) Blanching (heating at about 100°C) iii) Sterilization (heating above 100°C) You can find more details in Para 13.5 and 13.6. We can see a time and temperature combination during canning process. The canned food is most sterile when packed; hence the filled cans are exposed to temperature-time profile sufficient to give a safe product. In this process the heat is applied externally which will be the cause of product to lag the surface and bulk product temperatures. Therefore it is essential to estimate the temperature at the slowest heating point. It will also ensure that a product is not over processed. For example canned low acid foods having the pH greater than 4.5 is generally processed at a reference temperature of 121.1°C for 3 minutes at its slowest heating point. Thus, we can consider this thermal process of canned food as safe. If the temperature of thermal process varies, then time will also vary and it can be seen, from the literature. The time – temperature profile as follows: 1.2 min at 125°C; 3.9 min at 120°C; 12.2 min at 115°C; 38.7 min at 110°C; 122.2 min at 105°C and 386.5 min at 100°C.

13.7.2 pH or Acidity The processing temperature is generally governed by the pH or acidity of food product. The food having a lower pH than 4.6 or higher acidity is generally given low temperature heat treatment. While, food having a higher pH than 4.6 or lower acidity is given high temperature heat treatment. Food product can be classified based on pH or acidity as follows: 1. Low acid, pH 5.0 and higher: This class includes mostly non-acid vegetables and some fruits such as peas, beans, corn, asparagus and bael

Processing and Preservation fruit, papaya etc. All types of microorganisms including Clostridium botulinum can spoil these products. 2. Medium acid, pH 5.0-4.5: Vegetable mixtures, soups, which has partially acidic ingredients. All types of microorganisms also spoil these. The flat sour producing microbes is of importance. 3. Acid, pH 4.5-3.7: This group includes tomatoes, pears, pineapple, figs etc. These products can be spoiled by non-spoiling aciduric types, butyric anaerobic and thermophilic anaerobic. 4. High acid, pH 3.7 and below: This class includes citrus juices, grape fruits, rhubarb, berries, pickles and sauerkraut. The bacterial spores are easily destroyed in fruits at pH 3.0 than in vegetables at pH 5.0 and above. These microbial spores do not grow at pH below 4.5. Hence, pH 4.5 has been considered as dividing line between acid and non-acid foods. It is usually practice in canning process. A canned food product, having a pH below 4.5, can be processed at boiling water (100°C). But the food having pH above 4.5 is processed under pressure 0.7 to 1.0 kg/cm2 to raise the temperature more than 100°C. The heat inactivation of enzymes in food products is generally governed by time of heat application as well as by pH. Lowering of pH by addition of acids, control enzymatic browning. Acids, which are naturally found in plant tissues such as citric, malic, and ascorbic acids help in reducing enzymatic browning.

13.8 ROLE OF HEAT APPLICATION DURING PRODUCT PREPARATION

Preparation of jam, jelly, juices, sauces, candies etc require a substantial amount of heating. This heat treatment is essential for safe preservation of these products, is being discussed below.

13.8.1 Use of Heat in Jam and Jelly Production Let us define the jam and jelly, marmalade and preserves, and then we will discuss how heat is essential during their preparation. Jam: Jam is prepared by boiling whole fruit pulp with cane sugar (sucrose) to a moderately thick consistency without retaining the shape of the fruit. As per FPO specification 45 parts of fruit to each 55 parts of sugar, is used for preparation of jam. Jelly: Jelly is prepared by boiling fruit with water, expressing as water (pectin) extract), adding sugar, and concentrating to such consistency that gelatinization takes place on cooling. The best jelly is clear, sparkling, transparent and of attractive colour. It should retain the shape when removed from container or glass bottle. Marmalade: Marmalade is a clear jelly in which shreds of peel are suspended. It is generally prepared from citrus fruits. Preserves: cooking prepared fruit in sugar syrup until the concentration of sugar reaches 55 to 70 percent makes Fruit preserves. In this product, fruits

Processing by Heat Application retain its shape and are crisp rather than soft. Here also 45 parts of fruit for each 55 parts of sugar is employed. Heat in Jam and Jelly Production Jams Jams may be made from all varieties of fruits. Good, fully matured fruits are selected, washed, peeled. Thin skinned fruits do not require peeling such as Apricots, plums etc but stone can be removed by machine. Fruits should be boiled in a small quantity of water and steamed and passed through pulper and finisher to get the desired texture pulp. Hence, heating is required to boiling or cooking the fruit to make into pulp. Cane sugar or sucrose is added to the pulp in equal ratio for most of the fruits. But for sweet fruits of low acidity such as ripe peaches, sweet prunes and grapes less than equal weight and sugar is required. Boiling is an important heat treatment in the preparation of jam, jelly, marmalade and fruit preserves. Boiling or cooking is desirable, it causes intimate mixing of the fruit pulp and sugar. It partially concentrates the product by evaporation of excess moisture. Boiling is again dependent upon the firmness of fruits. Soft fruits in small lots can be concentrated to desired consistency as rapidly as possible. Other hard fruits are more resistant to heat application, may be boiled more slowly. Steam-jacketed stainless steal kettles are commonly used in commercial practice. Vacuum pan is also used to minimize heat damage to the product. Most jam should be concentrated to boiling temperature of 103 to 1050C. The end points of jams boiling vary with fruit varieties, amount of sugar and some other factors. However, the finished jam should show 65 to 68 percent soluble solids as determined by refractometer. Jelly The pectin, acid and sugar are essential to the preparation of a normal fruit jelly, out of which pectin is the most important. To make a jelly of excellent consistency, pectin, acid, sugar and water must be in the proper portion. If the fruit is deficient in any one of the constituents, which can be added from outside source. However, we should take proper care in selection of fruits. Fruits rich in pectin, acid and sugars and fully ripe are selected to impart good flavour and texture to the finish product. The pectin is precipitated as a hydrated colloid that forms a network of fibrils throughout the mass, binding the sugar into the gel. The concentration of sugar makes the texture stiffer. Acid causes the jelly to be the firmer by toughening the fibrils. This is the reason that these constituents must be in the right proportion viz. pectin 1%, sugar 60 to 65%, fruit acid 1% and water 33 to 38%. Washed fruits are cut into slices or crushed. Very juicy fruits like berries, do not require water, simply they crushed and heated to the boiling point for 2 to 3 minutes and juice or pectin extracted pressed out. Firmer the fruits like apple, guava, oranges are cut or crushed and water is added half to equal volume of the fruits. They are required to be heated to 20 min. Citrus fruits require two to three volume of water for each volume of fruits and require to be heated for 30 to 60 min. After addition of water, the fruit is boiled and then slowly put on simmering fire for the sufficient time (as mentioned above) to extract pectin, acid and

Processing and Preservation sugars. A clear fruit extract is filtered and tested for pectin content by alcoholic test. The equal amount of juice and alcohol is taken in a centrifuge tube, mixed well, centrifuging, and reading the volume of the sediment. The viscosity of the juice varies with the pectin content hence, the viscosity test by any standard method may be used as a guide for pectin content. Then fruit extract, sugar, acid are taken in an appropriate proportion. The whole mass is boiled to the end point. Boiling is one of the important steps in jelly preparation. The boiling dissolve sugar, causes mixing of sugar, acid and pectin to form jelly. During heat treatment certain organic compounds coagulates, which can be skimmed from the surface and their removal make jelly clearer. The principal purpose of boiling is to increase the concentration of sugar to the point where jellying will occur. End point can be measured accurately by use of a thermometer. The thermometer is inserted in the boiling juice. If the juice contains the proper portion of sugar, acid and pectin, the boiling point of the liquid will be a few degrees higher above the boiling point. At sea level the boiling point will be 104 to 1050C and corresponding to a concentration of 65 to 68 percent total solids in the jelly after cooling. This can be also measured by refractometer. Marmalade Marmalade preparation is similar to jelly with the difference that it contains pieces of fruit peel suspended there in. The principles of jelly making, apply also to the preparation of marmalade. Preserves Fruit preserves should retain the form of original fruit, either whole or cut fruit in clear sugar syrup of higher concentration. Fruit should not be overcooked or caramelized. Preserves are processed by three different methods viz. open-kettle one-period process, the slow open-kettle process and by vacuum cooking of preserves. Heat treatment is the essential part of these cooking and it influences the quality of the finished products. These three methods are briefly discussed below: Open kettle one period process: This is a simple method of preserve making and usually employed by housewives. In this process fruit is boiled in steam- jacketed kettles with sugar or in syrup until fruit is impregnated with thick or heavy density syrup. Soft fruits and berries should be boiled for short time. But the firm fruits like peach, pears, aonla require a long period to impregnated them with the syrup. Here, the end point can easily be determined by refractometer at 68 percent soluble solids or by a thermometer, which shows the boiling point about 104 to 105°C at sea level. It is a rapid method with low cost operation, but may result in serious injury to flavour and colour of the finished product. The slow open-kettle process: In this process the fruit is heated for short time on successive days in sugar syrup of progressively increasing sugar concentration. This will avoid undue injury, to the colour and flavour of the preserves. Initially fruit is boiled enough in syrup containing about 40 percent sugar. Then the mixture kept aside for 24 hours and then further boiled for 3- 4 minutes with the addition of 10 percent more sugar. This step is repeated

Processing by Heat Application until the product has the desired consistency and then placed in the final containers and sterilized. Vacuum cooking of preserves: The vacuum cooked preserves are of superior quality in respect of flavour and colour then the preserves made in an open cooker. The fruit preserves filled into glass jars or can at 88 to 960C and in commercial practice are generally pasteurized. Candies: Candied fruits or candies are prepared essentially of slowly impregnating the fruit with sugar syrup until the sugar concentration in fruit is so high enough to prevent spoiling. Repeated boiling and storage in syrups of progressively increasing concentration of sugar generally accomplish this. The fruit is washed and dried to remove excessive sugar from surface. Then it is coated with glaze of sugar and glucose syrup, called glazed or candies. We have learned that heat treatment is essential part of the preparation these products.

13.8.2 Use of Heat in Ketchup and Sauce Production Tomato ketchup:– Tomato ketchup (catsup, catchup) can be defined that it is a clean, sound product made from strained tomato juice, with spices, sugar, salt, vinegar, onion and garlic etc. It should contain not less than 12 percent tomato solids and 28 percent total solids. Sauce: It can be defined that the sauce is a clean, sound product made from properly prepared fruits/ tomatoes / peppers with spices, salt, sugar, ginger, onion, garlic etc. It should contain total solids from 12 to 25 per cent. For preparation of tomato ketchup the first step to prepare tomato juice or pulp for which fully ripe, red coloured fruits are taken. Heat application plays an important role in juice extraction from tomatoes particularly in hot break method. Tomatoes are cut or sliced and immediately heated to boil in their own juice for 3 to 5 minutes to facilitate juicing or pulping. The heat treatments help in the following ways: 1. The pectin present in the skin and seeds can be incorporated into the juice otherwise juice may separate into liquid and pulp.

2. Heat treatment inactivates/destroy the pectate enzymes otherwise hydrolyse the natural pectin present in tomatoes and resulting the thin juice.

3. Boiling tomatoes release the pectin and it thickens the pulp.

4. Boiling sterilizes the tomato juice partly, thereby controlling to some extent the growth of microorganisms, which may cause fermentation etc in the juice.

5. It also inactivates some oxidative enzymes, resulting better retention of vitamin C.

6. The cooking also releases the red colour present in the tomato skin.

7. Heat treatment yield higher juice recovery than cold method.

The tomato ketchup can be made from tomato juice or its concentrate or paste. Here we can say that for preparation of juice concentrate, paste or ketchup the

Processing and Preservation original tomato juice has to be concentrated by application of heat. Thus, heat treatment evaporates water resulting in concentration of juice. The concentration of tomato juice is carried in two ways i.e. by open cooking and cooking under vacuum.

In the open cooking – the tomato juice is cooked or boiled in aluminium vessel (Patila), at smaller scale. In large scale, the juice is heated mostly in stainless steel steam jacket – kettle to the desired consistency, which can be measured either by refractometer or specific gravity hydrometer. The open cooking is generally employed, but it has some disadvantages. During cooking the product is exposed to the oxygen in the air, which may destroy vitamin C and makes the juice brown. Sometimes edible oil is added to prevent foaming, boiling, over sticking or burning. It also helps in lessening oxidation.

In vacuum cooking – All the above defects are removed in vacuum cooking. In this method the juice is placed in vacuum pan and heated. Under vacuum, boiling takes at reduced temperature of 71°C. This method results in the superior quality of finished product with better retention of vitamin C and colour than open cooking. In the end vacuum is broken and the juice is sterilized by heating to 100°C for about 10 minutes.

Sauce can be made from Soyabean, mushroom, apple and mixtures of various other fruits. Thin sauce is mainly consists of vinegar extract of various flavouring materials like spices and herbs for example Soya sauce, Worcestershire sauce. Thick sauce is highly viscous and contains fruit pulp and more sugar. Their cooking is similar to tomato ketchup.

Similarly fruit juices such as apple juice, pineapple juice, orange juice etc. are also concentrated which can be seen in other blocks and all needed heat treatment.

Check Your Progress Exercise 3 " Note: a) Use the space below for your answer. b) Compare your answers with those given at the end of the unit. 1. How heat application works during peeling. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 2 Describe the use of heat application during fruit juice processing. ……………………………………………………………………………… ………………………………………………………………………………

Processing by Heat Application ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 3. Explain that the heat processing is greatly influenced by the pH or acidity of the product. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… 4. Describe the important role played by heat application in the preservation of jam and jellies. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………

13.9 LET US SUM UP

Fruits and vegetables are generally preserved by heat application. Heat treatments are in terms of blanching, pasteurization and sterilization, which also affect the fruit and vegetable compositions, microorganisms and enzyme activity. Heat treatment protects the processed food products from spoilage causing bacteria, moulds and yeasts as well as enzymatic changes.

Heat treatment also made processed food palatable but reduces some of the nutrients. If appropriate time – temperature used and storage at low temperature helps in reducing the nutrient losses. It also enhances their storage life.

Appropriateness of the factors such as salt, sugar, fat, and protein, besides time – temperature and storage temperature also influences the quality and nutrients retention in the processed products.

Heat treatment in various forms is employed during canning process, for peeling, blanching, syrup or brine preparation, filling, exhausting, sterilization etc.

Processing and Preservation Heat is also applied during product preparation so that jam, jelly, and marmalade etc are set properly. Heat application is also employed in processing of juice and beverage.

Thus appropriate time – temperature depending upon the product and presence of microorganisms and enzymes can manufacture processed products of good quality with proper retention of nutrients.

13.10 KEY WORDS

Pasteurization : process of heating food product to a specific temperature for a specific time to kill the most heat resistant vegetative pathogen.

Sterilization : A process to heat food product to a specific temperature for a specific time to kill the most heat resistant spore-forming organism.

D-value : It is also called “decimal reduction time” or “thermal death rate” and defined as time (in minutes) at a particular temperature (°C) required to kill 90 percent of a microbial population.

Canning : It is defined as preservation of foods in sealed containers and usually implies heat treatment as the principal factor in the prevention of spoilage.

HTST : High temperature short time.

13.11 ANSWERS TO CHECK YOUR PROGRESS " EXERCISES

Check Your Progress Exercise 1 1. Your answer should include the following points:

• Constituents include carbohydrate, proteins, fats, vitamins, minerals, fibre, water, flavouring compounds 2. Your answer should include the following points:

• Palatability • Storage life • Food borne diseases 3. Your answer should include the following points:

• Blanching - Leaching, thermal oxidation, product damage • Pasteurization - Enzymes • Commercial - Heating food enzymes • Sterilization 4. Your answer should include the following points:

• Different heat treatment terms are:

Processing by Heat Application UHT, Pasteurization, sterilization, commercial sterilization. 5. Your answer should include the following points:

• Temperature – time relationship • Environment (Nutrients, incubation temperature, state of growth and age, desiccation • Substrate composition (moisture, pH, other constituents)

Check Your Progress Exercise 2 1. Your answer should include the following points:

• Blanching • Pasteurization • Commercial sterilization 2. Your answer should include the following points:

• Degree of heat application / HTST • Time of heat treatment • Better nutrition, flavour • Minimum cooked taste if any 3. Your answer should include the following points:

• Heating at higher temperature • Complete elimination of microorganisms • Destruction of pathogenic, spoilage causing organism

Check Your Progress Exercise 3

1. Your answer should include the following points:

• Types of peeling • Heating medium – boiling water, steam, roasting, steam under pressure • Softening, cracking, loosening of skin

2. Your answer should include the following points:

• Heat application – in clarification and preservation of juice • Destruction of yeasts and moulds but not killing the spore forming bacteria • Time and temperature

3. Your answer should include the following points:

• Food classification based on pH and acidity

4. Your answer should include the following points:

• Mixing of fruit and sugar • Preparation of pectin or water extract for jelly making • Cooking point • Temperature of fruit and sugar mixtures

13.12 SOME USEFUL BOOKS

Processing and Preservation 1. Ammerman, G.R. (1957) The effect of equal lethal heat treatments at various times and temperatures upon selected food compounds. Ph.D. Thesis. Purdue Univ., W. L Lafayette, IN.

2. Arthey, D. and Ashurst, P.R. (2001) Fruit Processing. Nutrition, Products, and Quality Management. A.N. ASPEN Publication. Aspen Publishers, Inc. Gaithersburg, Maryland.

3. Ball, C.O. (1923) Thermal process time for canned foods. Bulletin of the National Resources Council, 7(1).

4. Frazier, W.C. and Westhoff, D.C. (1986) Food Microbiology. Tata Mc Graw-Hill Publishing Company Limited, New Delhi.

5. Karmas, E. and Harris, R.S. (1989) Nutritional Evaluation of Food Processing. An AVI Book, Published by Van Nostrand Reinhold Company, New York.

6. Richardson, P. (2001) Thermal Technologies in Food Processing. CRC Woodhead Publishing Limited, Cambridge England.

7. Shapton, D.A., Lovelock, D.W. and Laurita–Longo, R. (1971) The evaluation of sterilization and pasteurization processes from measurements in degrees Celsius (°C). J. Appl. Bacteriol. 34, 491-500.